Method for measuring coagulation of milk

ABSTRACT

A measurement method for determining the state of coagulation of raw milk in a production process for, for example, cheese or yogurt. According to this method, the state of coagulation of milk can be determined by placing a metal wire in the milk and, while feeding an electric current continuously or intermittently to the metal wire, measuring the temperature of the metal wire over a given period of time.

TECHNICAL FIELD

This invention relates to a measurement method for determining the stateof coagulation of raw milk in production processes of, for example,cheese and yogurt, and more specifically to a measurement method forthermally determining property changes which accompany the coagulationof milk.

The term "milk" as used herein means raw milks used primarily for theproduction of cheese and yogurt, such as whole milk, skimmed milk andreconstituted milk.

BACKGROUND ART

The coagulation stage of milk is the most important and fundamentaltreatment step in, for example, the production of cheese. The state ofcoagulation essentially governs the quality of the resulting finalproduct. Given these circumstances, the determination of the state ofcoagulation of milk has conventionally been carried out subjectively onthe basis of the experience of skilled technicians. On the other hand, anumber of measurement methods have also been invented to measure thecoagulation of milk by means of instruments. However these methods areaccompanied by such drawbacks as the milk coagulum has to be squeezed orpressed to cause it to change shape, and in some instances, cause wheyto be released. As a result, measurements are made on the coagulatedmilk in a state different from its normal state. Use of such measurementmethods are thus limited to research purposes, and they are impracticalfor actual application in the production of, for example, cheese.

DISCLOSURE OF THE INVENTION

Given the above-described situation, the present inventors haveconducted an investigation with a view toward developing a method formeasuring the coagulation of milk, which method may be applied to theproduction process of, for example, cheese. As a result, it has beenfound that the state of coagulation of milk can be determined bythermally investigating property changes which accompany the coagulationof milk.

Accordingly, an object of this invention is to provide a method formeasuring the coagulation of milk, which method is capable ofdetermining the state of coagulation without the need to apply externalpressure such as squeezing or pressing to the milk coagulum and can beapplied to the production process of, for example, cheese.

A special feature of the constitution of this invention resides in thatin the coagulation stage of milk, the state of coagulation is determinedby placing a metal wire in the milk and then measuring with time thetemperature of the metal wire while feeding an electric current eitherintermittently or continuously to the metal wire.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 exemplifies a sensor which consists of a metal wire to beemployed for measurements in the present invention.

FIG. 2 exemplifies the manner for using the sensor to measure thecoagulation of milk.

FIGS. 3 and 4 show the relationship between the temperature of the metalwire and the time, when a direct and constant electric current was fedintermittently and continuously to the metal wire, respectively.

FIGS. 5 and 6 illustrate the convection-starting time (tc) in raw milkand the degree of temperature drop (θc), respectively, due to convectionwhen a direct and constant electric current was intermittently fed inExample 1 of this invention.

FIG. 7 shows the time dependence of the temperature of the metal wirewhen a direct and constant electric current was also fed continuously.

FIG. 8 illustrates the relationship among the length (tr) of the 1ststage, the coagulating velocity (dθ/dt) of raw milk and the treatmenttemperature when a direct and constant electric current was continuouslyfed in Example 2.

BEST MODE FOR CARRYING OUT THE INVENTION

The metal wire to be placed in milk in the present invention should havea diameter of 0.01 mm-2 mm or so and is preferably made of platinum. Inorder to place such a metal wire into milk (raw milk) to be coagulatedand then to cause a current to pass therethrough, the method exemplifiedin FIGS. 1 and 2 of the accompanying drawings should be followed.

In FIG. 1, 1 indicates a platinum wire. Current-feeding terminals 2,3are connected to the two ends of the platinum wire, whereasvoltage-measuring terminals 4,5 are connected at suitable preselectedlocations on the platinum wire, preferably at locations apart by 1 cm ormore from their corresponding terminals 2,3. The thus-assembled unit,which is generally designated S in FIG. 1, is used as a measuring sensorin this invention along with a voltage measuring unit 7 which is shownin FIG. 2. FIG. 2 exemplifies the manner for using the above measuringsensor to measure the coagulation of milk. In the Figure, there areillustrated the sensor S, a current source (constant electric currentsource) 6, a voltage-measuring unit 7, a control unit 8, a display unit9 for time vs. temperature curves, a coagulation vat 10 for milk, rawmilk 11 and a conventional control system 12-14.

In the above-described embodiment of this invention, the sensor S isplaced in the raw milk (for example, skimmed milk) held in the vat 10and while feeding a current (usually, a direct and constant electriccurrent) either intermittently or continuously from the current source 6to the sensor, temperature variations of the metal wire, whichvariations occur due to coagulation of the raw milk, are measured over agiven period of time. This measurement of temperature variations iseffected by measuring the voltage of the metal wire by means of thevoltage-measuring unit 7, and the temperature variations are calculatedin accordance with the following equation:

    θ=(V/iR.sub.o -1)/α

where θ means the temperature (°C.) of the metal wire, V the voltage (involts) of the metal wire, i the electric current (in amps) of the metalwire, R_(o) the electrical resistance (Ω) of the metal wire at 0° C.,and α the temperature coefficient (1/° C.) of the electrical resistance.

Determination of the state of coagulation of the raw milk can then becarried out by measuring the display 9 of a logarithmic time vs.temperature curve (in the case of intermittent electric current feeding)or a time vs. temperature curve (in the case of continuous electriccurrent feeding) for the metal wire while feeding a current to the metalwire in the above-described manner. It is quite clear that the wire 1with its terminals 4, 5 and the voltage measuring unit 7 act as atemperature measuring structure. The same wire 1 is heated overterminals 2,3 and current source 6. Display 9 gives the temperature overa period of time (see FIG. 3).

The relationship between the temperature variations of the metal wireand logarithmic time and between the temperature variations and time areshown in FIG. 3 (in the case of intermittent electric current feeding)and FIG. 4 (in the case of continuous electric current feeding).respectively.

As can be seen in FIG. 3, convection occurs around the metal wire afterpoint A. Thus, the mechanism of heat transfer from the fine metal wiretoward the surrounding raw milk changes from conduction heat transfer toconvection neat transfer. The time of point A, i.e., theconvection-producing time (tc), becomes gradually longer as thecoagulation of the raw milk proceeds owing to treatment by, for example,rennet added to the raw milk. On the other hand, the degree oftemperature drop θc of the metal wire, which temperature drop takesplace in association with the occurrence of convection, becomesconversely less. Further, after point B, nonsteady-state convection,i.e., turbulence occurs.

When rennet is added to raw milk to cause it to coagulate,characteristic changes can be observed, as shown in FIG. 4, in thechange from an enzymatic reaction (1st stage) to a non-enzymatic changes(2nd stage) and in the elapsed change in the 2nd stage. These changes inthe 2nd stage show the state of coagulation of the raw milk. It may bementioned that κ (kappa)-casein (which takes part in the stabilizationof casein micelle in milk protein) which is locally present on thesurface of casein micelle of the raw milk is specifically degraded bychymosin contained in rennet in the 1st stage. In the 2nd stage, thecasein micelle, the hydrophobicity of which has increased due to thedegradation of κ-casein, reacts with calcium ions, thereby inducingcoagulation of casein micelle and coagulating the raw milk.

The electric current to be fed to the metal wire in the presentinvention is determined in accordance with the diameter of the metalwire. In the case of platinum wires having diameters of 0.03 mm and 0.1mm, for example, direct and constant electric currents of 0.05-0.2 A and0.5-1.0 A, respectively, are preferred.

No special limitation is imposed on the length of each metal wire.However, to obtain measurement accuracy, about 5-30 cm is preferred.

As has been described above, the state of coagulation of raw milk can bedetermined by detecting variations in the hydrodynamic characteristicsof the milk, which variations occur in association with the coagulationof the milk, while feeding an elecric current to a metal wire placed inthe milk, primarily by detecting variations in kinematic viscosity asvariations in the transfer of heat from the metal wire heated because ofthe feeding of the electric current thereto to the surrounding milk.

When a direct and constant electric current is fed to a metal wire, forexample, a platinum wire placed in raw milk, the temperature of themetal wire rises because of Joule heat. However, this temperatureincrease does not give any problem or inconvenience in conductingmeasurements in accordance with this invention, because its rise islimited. In other words, when the surrounding milk is heated by thethus-heated metal wire, differences occur in the density of the milk andconvection is soon produced. Since the quantity of heat carried away byheat transfer through convection will increase in proportion to thetemperature of the thus-heated metal wire, the temperature of the metalwire will reach an equilibrium value at a certain time point as long asthe milk is present in a sufficient amount when a direct and constantelectric current is fed continuously. Since the influence of radiationcan be ignored in the above case, the transfer of heat from the metalwire to the surrounding milk is effected by means of conduction andconvection. When milk is subjected to a coagulation treatment, theproportions of transfer of heat by means of conduction and of convectionin the transfer of heat are not constant over a given period of time.Since the kinematic viscosity increases as the coagulation proceeds, theproportion of the transfer of heat by convection decreasescorrespondingly.

Further explanation will be made in this matter. When a metal wireplaced in a liquid is one-step heated (heated not gradually butinstantaneously), heat is transferred from the metal wire to thesurrounding fluid by means of conduction alone during the timeimmediately following the heating (for example, 5 seconds when a directand constant electric current of 0.7 A is fed to a platinum wire of 0.1mm in diameter and 10.8 cm long fixed vertically in reconstitutedskimmed milk having an total solid content of 10% and a temperature of30° C.). As a result, as theoretically derived from Fourier equation ofheat conduction, the temperature of the metal wire increases linearlyrelative to logarithmic time. When the temperature of the metal wirehowever rises further and exceeds a certain critical value, convectionwill be produced around the metal wire (point A in FIG. 3) andthereafter hydrodynamic effects will become dominant. In other words,the velocity of heat transfer will become greater because of the effectsof the transfer of heat by means of convection. Therefore, the rate oftemperature increase of the metal wire will be reduced correspondingly,and the transfer of heat will then be changed to the transfer of heat byturbulence which is non-periodic convection (point B in FIG. 3), therebyallowing the temperature of the metal wire to reach substantially anequilibrium value. Since the time span from one-step heating to theoccurrence of convection increases gradually as the coagulation of milkproceeds, it is possible to measure the coagulation of the milk byobserving the convection-producing time to or the degree of thetemperature drop θc of the metal wire due to convection over a givenperiod of time while feeding a direct and constant currentintermittently to the metal wire in the course of the coagulationtreatment of the milk.

On the other hand, when a direct and constant electric current is fedcontinuously to the metal wire, the coagulation of the milk can bemeasured as variations in the equilibrium temperature of the metal wireover a given period of time. In other words, when the kinematic velocityincreases and the proportion of the transfer of heat by means ofconvection in the heat transfer decreases along with the progress ofcoagulation of the milk as mentioned above, the equilibrium temperatureof the metal wire will rise significantly, leading to provision of thecharacteristic time vs. temperature curve shown in FIG. 4.

Among the methods of this invention, according to the method featuringthe continuous feeding of a current to a metal wire, it is feasible notonly to measure the coagulation of milk in a qualitative fashion, butalso to determine the milk-coagulating capacity of rennet from thelength tr of the 1st stage in FIG. 4 when the coagulation treatment ofmilk is effected using rennet. The milk-coagulating capacity of rennetvaries in a complicated pattern, depending, for example, on the type andconcentration of the rennet, the conditions of the milk and thetreatment temperature. In addition, the velocity at which milkcoagulates has a major effect on the texture of cheese and can also beestimated as a relative value from the rate of temperature variation inthe 2nd phase of FIG. 4 (for example, the gradient dθ/dt of the curve atpoint D). On the analogy of heat transfer, the kinematic viscosity ofthe milk coagulum can also be immediately obtained from the degree oftemperature increase of the metal wire. By feeding a direct and constantelectric current continuously to the metal wire during the rennettreatment of milk, it is possible to measure, from the characteristiccurve of FIG. 4, the entire process of the milk coagulation in acontinuous, quantitative and non-destructive manner. Further, thecompletion of the coagulation can be determined from point E, at whichthe temperature of the metal wire becomes constant relative to the time.

As mentioned above, the present invention permits precise determinationof the state of coagulation by thermally measuring changes in thecharacteristic properties of raw milk, which changes occur along withthe coagulation, on the basis of temperature changes in a metal wireplaced in the raw milk over a given period of time when an electriccurrent is fed to the metal wire. Accordingly, the present invention cansolve the drawbacks mentioned above which are encountered inconventional experience-dependent judgements or in instrument-dependentmeasurements used in laboratories.

According to the present invention, it is also possible to measure someparameters which pertain to the hydrodynamic characteristics whichaccompany the coagulation of raw milk. It is thus possible to determinewithout fail a very soft coagulation state (for example, the coagulationof yogurt) which has hitherto been considered difficult to determine.

The sensor used for measurement in this invention is in principlecomposed of a single piece of metal wire. It thus has such advantagesas, even when used in an actual production process, it is free from theproblems involved with conventional instruments in washing and becausethe physical quantity actually measured is the voltage of the metalwire, it can be used directly as a signal for automatic control of therennet treatment of milk.

The present invention will be described in further detail by thefollowing Examples:

EXAMPLE 1

Placed in a cylindrical flask 8.4 cm in diameter and 18 cm high was asample which had been prepared by adding 0.03% of rennet to 1 liter ofreconstituted skimmed milk having an total solid content of 10% and atemperature of 30° C. Along the central axis of the flask, a platinumwire (R_(o) =1.3974 Ω; α=3.817×10⁻³ 1/° C.) 0.1 mm in diameter and 10.8cm long was placed and fixed.

Then, a direct and constant electric current of 0.7 A was fedintermittently for 1 minute every 5 minutes after the addition of rennetto the platinum wire to obtain a logarithmic time vs. temperature curve.When the convection-producing time tc and the degree of temperature dropθc due to the convection (30 seconds after the feeding of the current),corresponding to point A in FIG. 3, were measured, the tc increasedsignificantly and the θc decreased significantly as the coagulation ofthe reconstituted skim milk proceeded (see FIGS. 5 and 6 where thecircles show actual experimental points through which the curves showncan be generated).

Using the same sample, a direct and constant electric current of 0.7 Awas continuously fed to obtain the characteristic curve shown in FIG. 4.It was able to measure, in about 20 minutes after the addition ofrennet, the state of change from the 1st stage, i.e., the enzymaticreaction, to the 2nd stage, i.e., the coagulation of casein micelle (seeFIG. 7). Separately, a characteristic curve was also determined on asample as a control which did not have rennet added. Changescharacteristic to the coagulation of milk were not observed.

EXAMPLE 2

Using a sample which had been obtained by adding 0.09% of rennet to 1liter of reconstituted skim milk having an total solid content of 8.5%and a temperature of 30° C.-45° C., an electric current was continuouslyfed under the same conditions as those employed in Example 1 todetermine the length (tr, min.) of the 1st stage and the coagulationvelocity (the gradient dθ/dt, °C./hr. at point D in FIG. 4). As aresult, the time tr required for the reaction in the 1st stage decreasedexponentially as the temperature of the rennet treatment rose andsubstantially reached equilibrium at 40° C. On the other hand, thetemperature-rising velocity dθ/dt which is considered to correspond tothe coagulation speed increased linearly relative to the treatmenttemperature (see FIG. 8). The findings resulting from these measurementsconformed with findings experienced in connection with changes in theproperties of milk due to rennet treatment.

Although each of the above Examples relates to coagulation by rennet,measurement can be made in the same manner when coagulation is effectedby using a microorganism rennet (for example, Mucor pusillus).

We claim:
 1. A measurement method for determining the state ofcoagulation of milk in production processes of milk products such ascheese and yogurt, which method comprises placing, as a measuringsensor, an assembled probe unit for insertion in the milk consisting ofa metal wire having a preselected length, current-feeding terminals andvoltage-measuring terminals connected to preselected spaced locations onsaid metal wire, selectively supplying an electric current eitherintermittently or continuously through the metal wire and, while theelectric current is being supplied therethrough, measuring and recordingthe degree of temperature increase of the metal wire through themeasuring sensor over a given period of time thereby determining thestate of coagulation of the milk in a continuous, quantitative andnon-destructive manner.
 2. A measurement method as claimed in claim 1,wherein said assembled unit, as a measuring sensor, is composed of ametal wire, the current-feeding terminals connected to two ends of themetal wire and the voltage-measuring terminals selectively connected totwo other locations of the metal wire.
 3. A measuring method, as claimedin claim 2, including adding rennet to the milk to cause it to coagulateduring the continuous supply of electric current.